The present invention relates to catalytic cracking of hydrocarbons, and more particularly relates to a process for reducing coke on spent catalyst during catalytic cracking.
One of the problems that has plagued the catalytic cracking art, and which has heretofore defied satisfactory solution, is the formation of "coke" during the cracking process which contaminates the catalysts and inhibits their effectiveness.
Regeneration of the contaminated or spent catalyst has proven to be a complex procedure requiring costly equipment. Heretofore, the "coke" had to be burned from the catalyst periodically. However, cracking catalysts in use today are susceptible to heat damage at temperatures which can develop during combustion. Such damage is aggravated by the presence of water vapor at high temperatures which is formed during the combustion of the carbonaceous material contaminating the catalyst. Gas purging or steam stripping of catalyst is usually employed to partially remove entrained hydrocarbons that may be volatized and recovered before the catalyst moves to the regeneration zone. Any hydrocarbon or coke not removed before regeneration is lost due to combustion which decreases product yield.
The problems associated with the burning of coke on the catalyst make it necessary to employ arrangements which dissipate the heat formed during regeneration. Heat transfer coils and heat exchangers have been utilized as expedients for this purpose.
In addition, there have been numerous prior art attempts to improve the reaction step in an effort to reduce the formation of coke on the catalyst and accordingly reduce the burden on the regeneration portion of the process.
One prior art approach to the problem involves prehydrogenation of the charge-stocks prior to the cracking operation. Another approach involves blending high hydrogen transfer hydrogen-donor-type feedstocks with the charge stock so that the catalytic cracking process is carried out in the presence of a "hydrogen donor". See U.S. Pat. Nos. 3,413,212 and 3,533,936.
European Patent Application No. 0045178 described the conversion of carbonaceous material to liquids when contacted with vapor phase hydrogen donors for short contact times (1-10 seconds) at 1000.degree. F. and atmospheric pressure. The hydrogen donors are nitrogen compounds such as 1,2,3,4-tetrahydroquinoline; 1,2,3,4-tetrahydroisoquinoline; 1,2,3,4-tetrahydrocarbazole and indoline.
U.S. Pat. No. 3,413,212 discloses the use of zeolitic fluid catalytic cracking unit catalyst to promote reaction of water with deposited hydrocarbons. The same effect was not observed with inert stripping atmospheres such as nitrogen, argon or helium.
I. P. Fisher, Fuel, Vol 65, p. 473-79 (April, 1986) reported that microactivity apparatus results indicate that "additive" and "catalytic" coke yields from vacuum tower resid were insensitive to feed dilution with a hydrogen donor. The hydrogen donor was hydrotreated FCU LCO containing 44% substituted tetralins. It was also reported that exposure to stripping steam after regeneration increases activity and selectivity of zeolytic catalysts, and that long exposure (1-10 minutes) of spent catalyst to steam increases catalytic activity and increases selectivity toward higher naphtha and lower coke yield.
However, the art has yet to provide a satisfactory solution to the problem. One of the reasons for the prior art failure is that heretofore, coke was believed to be a stable carbon material. It has now been found that a portion of the coke is actually an immediately reactive, rapidly polymerizing organic complex, which, if contacted on spent catalyst with a hydrogen donor within a very short, critical time period after passing out of the primary reactor zone and into the stripper section of a catalytic cracking unit or to a separate hydrogen donor-stripper custom assembly, can readily be removed therefrom at normal processing temperatures and conditions.
Thus, the present invention fulfills a longstanding need.